This invention relates to an image processing device for adjusting the density of an image and an image reading device having the image processing device incorporated therein.
As image input means for such digital devices as copying machine, facsimile, film scanner, OCR (optical character reader), optical filing system, and various display devices which form, memorize, and display images by the combination of dots (pixels) of a fixed size, an image reading device (image reader) which reads an image on a given original document in the form of an aggregate of distinctly divided pixels has been in popular use.
Generally, the image reading device is so constructed as to operate by a procedure which comprises reading the image of a given original document by the use of a one-dimensional image sensor extending in the direction of main scanning, converting the analog signals of the individual pixels obtained by the reading into pixel data in the form of multivalue digital signals, subjecting the pixel data resulting from the conversion to image processing, and producing binary image data corresponding to the individual pixels.
The image processing which is performed in the image reading device is known in numerous forms including edge enhancing and smoothing processings for the improvement of image quality, trimming and negative-positive inverting processings for the editing of an image, density conversion processings for effecting tone compensation in accordance with the .gamma. characteristics of an externally connected device (image reproducing means such as a printer) to intensify the density gradient during the reproduction of an image, and binarization processings for the conversion of multivalue data in accordance with the density of each of the pixels of an original document into binary data, for example.
The so-called lookup table method is widely adopted for the density conversion processings. To be specific, the compensation of image data is accomplished by installing an ROM (read only memory) having stored therein a conversion table which is a collection of necessary compensation image data and consequently enabling such compensation image data as are stored at addresses answering the numerical values of input image data to be read out in the form of output image data D.
With respect to binarization processing, the so-called simple binarization processing which resides in comparing a multivalue level of one given pixel with a fixed threshold level is suitable for handling such linear images as characters from the point of view of the quality of a reproduced image and the pseudogradient processing which resorts to the dither method or the error diffusion method is suitable for handling such intermediate tone images as photographs. Where an original document which contains both linear images and intermediate tone images in a mixed state is to be read, therefore, the simple binarization processing and the pseudogradient processing are used as switched in accordance with the attributes of a given image which decide between a linear image and an intermediate tone image (hereinafter referred to occasionally as "binarization attributes").
Incidentally, the conventional image reading device has performed the density conversion processing by the use of one and the same conversion table. In other words, the contents of the tone compensation performed on image data have been set and fixed so as to exalt the reproducibility of images relative to images of standard density distribution.
Thus, in the binarization by the pseudogradient processing, the conventional image reading device suffers from impairment of the reproducibility of white and black parts of an image having a large density contrast and conversely entails the problem of reproducing such an unnatural image as a binary image from an image having a small density contrast.